
where \(J/\psi \)  and \(m_{J/\psi }\)  denote the field and the mass of the \(J/\psi \)  meson, respectively. \(f_{J/\psi }\)  is
the decay constant of the vector meson \(J/\psi \) . Using the experimental decay width (\(\Gamma _{J/\psi \rightarrow {}e^{+}e^{-}}=5.529\)  KeV)
and the masses of the \(m_{J/\psi }=3096.916\)  MeV [1]}, one obtains \(e/f_{J/\psi }=2.209\times {}10^{-2}\)  [2]}.
Moreover, the coupling constant \(g_{\gamma {}\eta _1J/\psi }\)  can be determined by the decay width of the \(J/\psi \) 
in \(\gamma {}\eta _1\)  channel. However, the experimental value of the width \(\Gamma _{J/\psi \rightarrow \eta _1\gamma }\)  at present is unclear.
It can only be determined by the theoretical calculation. Fortunately, the branching fraction Br(\(J/\psi \rightarrow \gamma \eta _1(1855)\) )
is estimated to be about \(6.2\times {}10^{-5}\)  [3]}. And the decay width of \(J/\psi \)  into \(\gamma \eta _1(1855)\)  is
\(\Gamma _{J/\psi \rightarrow \gamma \eta _1(1855)}=5.741\)  eV by using the \(J/\psi \)  total width \(\Gamma _{J/\psi }=92.6\)  KeV [1]}.
With this value and Eq. REF , we can obtain
\(g_{\gamma {}\eta _1J/\psi }&=\sqrt{\frac{96\pi {}m^2_{\eta _1}m^5_{J/\psi }\Gamma _{J/\psi \rightarrow \gamma \eta _1(1855)}}{(m^2_{J/\psi }-m^2_{\eta _1})^3(m^2_{J/\psi }+m^2_{\eta _1})}}\nonumber \\&=7.483\times {}10^{-4}.\) 
